Magnetic tunnel junctions used in magnetic random access memory (MRAM) can also be used as one-time-programmable (OTP) memory elements. These have the advantage that they can store information even during high temperature packaging, thus allowing data stored at wafer-level test, such as redundancy addresses, to be retrieved after packaging.
The information is written to the OTP memory elements by applying a large voltage to the junction, thus breaking it down, causing it to be electrically shorted. However, MRAM bits are designed to have high breakdown voltage, to allow the main multi-time-programmable (MTP) MRAM bits to be written many times without accidentally breaking down.
It would be advantageous to use the same type of bits, processed at the same time, for both MTP and OTP applications. One way that has been proposed to do this is to make the OTP bits smaller, which decreases their breakdown voltage. See, for example, Jan et al., “Demonstration of an MgO Based Anti-Fuse OTP Design Integrated With a Fully Functional STT-MRAM at the Mbit level,” 2015 Symposium on VLSI Technology Digest of Technical Papers, pp. T164-T165 (June 2015). However, this is only a small effect, and requires printing the devices so small that there will be manufacturing control issues.
Thus, alternative methods for co-processing MTP and OTP MRAM bits would be desirable.